Conventional sintering of alumina involves hydrogen or vacuum firing. For example, U.S. Pat. No. 3,026,177 to St. Pierre et al. disclosed the production of a transparent alumina body by subjecting a compact of C., to a first firing in hydrogen at 1650.degree. C. C., to 1750.degree. C. to effect removal of gas-containing pores C., the body, and subjecting the body to a second firing in hydrogen at 1800.degree. C. to 2000.degree. C. for not less than 15 minutes to remove additional pores and improve the transparency of the body. The dew point of hydrogen was not stated.
U.S. Pat. No. 3,026,210 to Coble disclosed the production of a high density alumina body having substantial transparency by forming a mixture of alumina powder and a small but effective amount up to 0.5 weight percent of magnesia powder into a compact, and firing the compact at 1700.degree. C. to 1950.degree. C. in vacuum or hydrogen for 1.5 to 4 hours. The dew point of hydrogen was not stated.
The effect of atmosphere on sintering of alumina was studied by Coble, "Sintering Alumina: Effect of Atmospheres," Journal of the American Society 45 [3]123-127 (1962). It was concluded that the shrinkage rate was not affected by changing the atmosphere from minus 70.degree. F. dew point oxygen to minus 70.degree. F. hydrogen. However, whether the degree of transparency was affected or not was not stated. It was stated that alumina powder compacts containing 0.25% weight magnesia sintered to theoretical density if the ambient atmosphere was hydrogen or oxygen, but would not sinter to theoretical density in air, nitrogen, helium, or argon atmospheres.
U.S. Pat. No. 3,377,176 to Wolkodoff et al. disclosed the production of high-density alumina body by sintering an alumina compact doped with 0.1 weight percent of magnesia and yttria at 1400.degree. C. to 1550.degree. C. in hydrogen or vacuum for 4 hours. The dew point of hydrogen was not stated.
U.S. Pat. No. 3,905,845 to Kobayashi et al. disclosed the manufacturing of polycrystalline translucent alumina body by sintering an alumina compact doped with 0.05 to 0.5 weight percent of yttria, 0.05 to 0.5 weight percent of lanthana, and 0.01 to 0.1 weight percent of magnesia in vacuum or a reducing atmosphere at 1600.degree. to 1800.degree. C. for 5 hours. The abstract said reducing atmosphere, but in all the examples cited, dry hydrogen was used, except that in one example, vacuum was used.
U.S. Pat. No. 4,285,732 to Charles et al. disclosed the production of a translucent alumina body by sintering an alumina compact doped with magnesia (0.3 to 0.15 weight percent) and an additive selected from ZrO.sub.2 (0.002 to 0.07 weight percent) and HfO.sub.2 (0.03 to 0.12 weight percent) in hydrogen at 1750.degree. C. to 1950.degree. C. for 3 hours. It was stated in this patent that the particular dew point of hydrogen was not critical, "in practice, it may range up to about 30.degree. C., and generally dew points from about minus 10.degree. C. to about plus 20.degree. C. are utilized." The dew point of the hydrogen was measured on the downstream end of the furnace. In the examples listed in the patent, the dew points of the hydrogen used ranged from minus 30.degree. C. to plus 20.degree. C., and the dew point of hydrogen did not appear to be particularly important.
U.S. Pat. No. 3,711,585 to Muta et al. described the use of magnesia (up to 0.1 w/o) and yttria (up to 0.25 w/o) as sintering aids for fabrication of alumina having high in-line transmittance at reduced sintering temperature and time as compared with using only magnesia as a sintering aid. The sintering atmosphere was selected from the group consisting of hydrogen and vacuum. The dew point of hydrogen was not stated.
U.S. Pat. No. 3,834,915 to J. J. Cleveland and C. W. Fritsch disclosed a translucent alumina composition, consisting of from about 99.8% to about 99.95% aluminum oxide, from about 0.025% to about 0.10% magnesium oxide, and from about 0.025% to about 0.1% of ytterbium oxide, sintered in hydrogen. The dew point of hydrogen was not stated.
U.S. Pat. No. 3,846,146 to B. J. Hunting et al. disclosed the use of magnesium oxide and calcium oxide as sintering aids for fabrication of transparent alumina in vacuum or hydrogen-containing atmospheres. However, the examples listed in the patent involved a hydrogen atmosphere of which the dew point was not stated. The "hydrogen-containing" atmosphere obviously referred to hydrogen gas.
British Pat. No. 1,443,741 to F. Puskas disclosed a cold flow technology for making shapes of translucent alumina containing a metal or metal compound comprising magnesium, cobalt, iron, calcium, nickel, molybdenum and tungsten sintered in a hydrogen gas atmosphere or in a hydrogen plasma. The dew point of hydrogen gas or plasma was not stated.
U.S. Pat. No. 4,373,030 to M. Kaneno and T. Kajihara disclosed a polycrystalline translucent alumina body doped with low level of MgO(0.01 to 0.1 w/o), La.sub.2 O.sub.3 (0.001 to 0.05 w/o), and Y.sub.2 O.sub.3 (0.001 to 0.05 w/o) and sintered under vacuum or either atmosphere of hydrogen gas (H.sub.2) or dissociated ammonia gas. (75% H.sub.2 and 25% N.sub.2) under oxygen concentration of 10.sup.-15 to 10.sup.-25 atm. The oxygen concentration was restricted to this range because of (1) trapping of oxygen inside the pores when the oxygen concentration was greater than 10.sup.-15 atm., and (2) evaporation of Al.sub.2pl O.sub.3 in the form of lower oxides when the oxygen concentration is less than 10.sup.-25 atm., as stated in the patent.
U.S. Pat. No. 4,222,978 to I. Oda and K. Maekawa disclosed a method for producing polycrystalline translucent alumina doped with 0.01 to 0.2 w/o MgO, 0.01 to 0.2 w/o La.sub.2 O.sub.3, and 0.01 to 0.2 w/o Y.sub.2 O.sub.3, by a first firing under vacuum or in an atmosphere of hydrogen gas or dissociated ammonia at 1300.degree. C. to 1500.degree. C. plus a second firing at 1650.degree. C. to 1900.degree. C. for one to 15 hours with a heating rate of greater than or equal to 200.degree. C/h for the temperature rise from 1400.degree. C. to 1700.degree. C., to achieve an average grain size of greater than or equal to 20,.mu.m and an average surface roughness of less than or equal to one-tenth of the maximum grain size with a high in-line transmission. However, the dew point of the hydrogen gas or dissociated ammonia gas was not stated.
U.S. Pat. No. 4,204,874 to S. Yamada disclosed a light-transmitting, polycrystalline alumina containing 0.04 to 1.5 w/o MgO and 0.2 to 3.5 w/o Cr.sub.2 O.sub.3 doped with 0.1 to 1.0 w/o SrO and sintered at 1650.degree. C. in vacuum or hydrogen. The dew point of hydrogen was not stated. In example 1 of the patent, ammonia cracked gas was used. The dew point of the ammonia cracked gas was not stated.
U.S. Pat. No. 3,792,142 to K. Kobayashi et al. disclosed a method of manufacturing a polycrystalline translucent alumina doped with 0.05 to 0.5 w/o Y.sub.2 O.sub.3, 0.05 to 0.5 w/o La.sub.2 O.sub.3, and 0.01 to 0.1 w/o MgO and sintered at 1600.degree. C. to 1800.degree. C. in vacuum or a reducing atmosphere such as hydrogen or dissociated ammonia. It was stated in the patent that it was necessary to use a nonoxidizing atmosphere for sintering. Hydrogen gas or ammonium decomposed gas for use in sintering was subjected beforehand to a drying step in which the gas was brought into contact with an activated alumina. In fact, in the five examples listed in the patent, four involved dry hydrogen and one involved vacuum atmosphere for sintering. Ammonium decomposed gas for sintering was not evident in the examples.
British Pat. No. 1,252,851 disclosed a method of manufacturing a transparent alumina, comprising alumina added with 0.05 to 0.5 w/o Y.sub.2 O.sub.3 and/or BeO, 0.05 to 0.5 w/o La.sub.2 O.sub.3 and/or CaO, and 0.01 to 0.1 w/o MgO, sintered at 1600.degree. C. to 1800.degree. C. in a vacuum or in a reducing atmosphere. In the examples of the patent, dry hydrogen was used.
U.S. Pat. No. 3,311,482 disclosed transparent bodies of aluminum oxide with an average grain size of less than 10 .mu.m doped with 0.25 w/o MgO and sintered at 1700.degree. C. to 1950.degree. C. in an atmosphere substantially free of gases having a molecular weight substantially greater than that of hydrogen. It implied that dry hydrogen was used.
U.S. Pat. No. 4,364,877 to R. Clement and F. Gugliermotte disclosed a continuous process of sintering of translucent polycrystalline alumina doped with 0.05 w/o MgO and/or rare earth oxides at 1820.degree. C. to 1970.degree. C. in a hydrogen-rich atmosphere. The atmosphere was either pure hydrogen or hydrogen to which were added "small" amounts of oxygen and/or CO and/or N.sub.2 and/or water vapor. It was not clear as to how small the amounts of the added gasses were. However, it implied that wet hydrogen gas containing nitrogen would be suitable.
U.S. Pat. No. 4,762,655 to Rhodes, Wei, and Fryburg disclosed a method of sintering translucent alumina in an atmosphere containing nitrogen and an amount of hydrogen greater than or equal to about 2.5 volume percent and less than 75 volume percent for a period of time sufficient to produce a translucent polycrystalline alumina body.